Half of young drivers who were killed in car crashes in the United States had consumed alcohol, marijuana or both. Out of the 7,191 fatal accidents studied, 36.8 percent were under the influence of alcohol, 5.9 percent used only marijuana and 7.6 percent used both substances.

Image via My Fluorish Magazine.

Researchers analyzed accidents involving drivers between the ages of 16 and 25 from the states of California, Connecticut, Hawaii, Illinois, New Hampshire, New Jersey, Rhode Island, Washington State and West Virginia. They chose these states because they routinely perform toxicological tests on the blood or urine of drivers killed in car accidents.

The first interesting statistic that came up is that out of all the crashes involving drivers under the age of 25 in these states, more than half took place in California! In the period between 1999 and 2011, 54 percent of these crashes occurred in California. At a first glance that’s quite a shocking statistic, but the reality of it is that California’s population is about as big as the rest 8 states combined… so it’s not strange after all.

But the substance consumption was the real shocker. The fact that over 50 percent of drivers killed were not sober is if surprising, then clearly worrisome. Dr. Katherine Keyes and her team knew that alcohol is one of the main causes of traffic accidents in the US (and worldwide), and they wanted to see if there are any change of patterns in substance consumption. In other words, if people over 21 consume more alcohol and marijuana (legally) than people under 21.

They found that alcohol consumption indeed increased by 14 percent for those over 21, but the pattern didn’t carry on to marijuana consumption. After reaching 21, simultaneous consumption of both substances increased slightly, but it’s not clear if it has any statistical relevance.

“Taken together, we found no significant substitution effect between alcohol and marijuana. Rather, increased availability seems to increase the prevalence of concurrent use of alcohol and marijuana,” says Dr. Guohua Li, co-author of the study and Director of the Centre for Injury Epidemiology and Prevention at Columbia University.

Recently, several states have legalized or decriminalized cannabis, and researchers wanted to see if availability of marijuana had any noticeable effects; it didn’t.

“Given the rapid changes currently underway in marijuana availability and permissibility in the US, understanding the effects of drug control policies on substance use behavior and adverse health outcomes, such as fatal motor vehicle crashes, has never been more important,” says Dr. Keyes.

Journal Reference: Keyes. K.M. et al (2014). Effects of Minimum Legal Drinking Age on Alcohol and Marijuana Use: Evidence from Toxicological Testing Data for Fatally Injured Drivers Aged 16 to 25 Years, Injury Epidemiology. DOI 10.1186/s40621-014-0032-1.